docs: def argmax(self):
          """Index of the maximum, ignorning nans."""
          if "argmax" not in self.attrs.keys():
  
              def f(dataset, s):
                  arr = dataset[s]
                  try:
                      amin = np.nanargmax(arr)
                  except ValueError:
                      amin = 0
                  idx = np.unravel_index(amin, arr.shape)
                  val = arr[idx]
                  return (tuple(i + (ss.start if ss.start else 0) for i, ss in zip(idx, s)), val)
  
              chunk_res = self.chunkwise(f)
              idxs = [i[0] for i in chunk_res.values()]
              vals = [i[1] for i in chunk_res.values()]
              self.attrs["argmax"] = idxs[np.nanargmax(vals)]
          return tuple(self.attrs["argmax"])